This invention relates to a new calibration method useful in lumped capacitance measurement of complex permittivity at HF, VHF and UHF frequencies, and in particular to a rapid and accurate method for calibrating an instrument for measuring reflection coefficients to calculate complex permittivity of dielectric materials.
The use of lumped capacitors for measurement of permittivity is not a new technique. The method has been employed at frequencies from direct current to as high as 4,000 MHz with very satisfactory results. The technique is inherently broadband because it does not depend upon resonant structures or propagation in frequency selective devices such as waveguides. The method when combined with computer controlled network analysis provides a powerful tool for studying the time course of dielectric dispersion in biological samples. This invention is related to U.S. patent application Ser. No. 938,625 entitled "An Electromagnetic Method for Non-invasive Analysis of Cell Membrane Physiology and Pharmacology" by L. E. Larsen, and John H. Jacobi, filed in the U.S. Patent and Trademark Office on even date herewith.
In the case of the lumped capacitance method, it is necessary to determine the admittance of the capacitor when filled with air and the complex propagation constant of the transmission line between the input connector and the sample holder in order to calibrate the instrument. The prior method for determining these constants involved calculating the capacitance value for the sample holder by measuring its dimensions, and determining the transmission line characteristics by removing the capacitor and replacing it with a standard of known reflection coefficient, typically a short.
By utilizing the calibration technique of this invention however the design or dimensions of the chamber, with the exception of its maximum dimension as will be subsequently explained, are not important and need not be measured. Therefore, the calibration technique of this invention allows great flexibility in design of the capacitance chamber. In addition, the full calibration can be completed in less than two minutes time. Therefore bad connectors or leaks in the chamber may be detected before embarking upon the experiment. Finally, bias errors in measurement of the phase of the sample holder reflection coefficient are canceled out in the calibration calculations according to the method of this invention. Accordingly, only relative phase measurements are necessary.